Licensed from Columbia University Press
In the case of the Solar System it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos had masses of about 1022 to 1023 kg and were a few thousand kilometres in diameter. Over the course of about 100 Ma they were involved in giant impacts with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually only a handful of embryos were left, which collided to complete the assembly of the planets proper.
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center; while lighter elements rose to the surface; such a process is known as planetary differentiation. Composition of some meteorites show that differentiation took place in some asteroids.
The giant impact theory proposes that Earth's Moon formed from a colossal impact of a hypothetical protoplanet, named Theia, with Earth early in the solar system's history.
References
This article is licensed under the GNU Free Documentation License.
Last updated on Saturday July 12, 2008 at 06:29:04 PDT (GMT -0700)
View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation
Copyright © 2008, Dictionary.com, LLC. All rights reserved.